Pool updates + other misc tweaks.

* Affix the theme name to the cached CSS/JS filenames to avoid loading the old cached file right after switching themes.
* Video thumbs should keep the aspect ratio when resizing now, rather than always resizing to the max thumb size.
* Added a prev link for pools, moved the nav creation for it (and the next link) over to theme.php & moved the CSS from themes/default to style.css
* Option to autoincrement pool order when post is added. (Rather than always using 0)
* Pool Navigation + Manage Pool should now always show on related pages (I.E pool/*).
* Optional setting for the pool: metatag to set image order (Via pool:ID:ORDER) (This overrides the default setting if used)
* lastcreated option for the pool: metatag, which adds the post to the last pool the user created.